Tensile strength is an important mechanical characteristic of a brittle material, such as rock. Conventionally, in a tensile test for directly test the tensile strength of rock, a test machine is used which comprises a sample clamping device as shown in FIG. 1. The clamping device is supported by a bottom frame of the test machine 1 and comprises an upper clamper and a lower clamper. Each of the upper and lower clampers comprises a connection member 12 or 2 connected with the test machine, an end cap 9 or 7 for holding a sample 8, and a chain 11 or 3 for connecting the connection member with the end cap. The lower clamper, for carry a part of the sample which is broken after test and the end cap 7, further comprises a supporting means composed of a rigid sleeve 4 and a backing plate 6. Connecting pins 10 and 5 are shown for attaching the chains 11 and 3 to the end caps 9 and 7 respectively. During the tensile test, there is a clearance between the backing plate 6 and a recessed sample-holding part of the lower end cap 7, and thus the rigid sleeve 4 cannot effectively support the weights of the backing plate 6 and the lower end cap 7 during the whole test. When a tensile force in the sample reaches its maximum or peak level, the sample is broken abruptly under the weights of a lower part of the sample (the part under a broken plane to be created in the sample), the lower end cap and the lower chain, and thus the mechanical behavior after the maximum or peak tensile force is reached cannot be measured.